As is well known in the art, the successful implementation of intelligent (self-adjustable) optical imaging systems requires devices capable of controllably changing their optical properties. One of the most important optical functions required to be adjustable is the focusing capacity and the focusing distance. Those properties are essential, for example, for the manufacturing of high quality cell phone cameras, storage/reading systems and adjustable glass of vision systems.
In modern high performance optical imaging systems, the optical zoom is obtained by the use of a mechanical movement. As a result, such imaging systems are of a relatively large size (in order to accommodate a motor, for example), heavy and generally have a slow zoom time (in the order of seconds). Several approaches to replacing the electro-mechanical zoom have been explored, including the use of liquid crystal (LC) technology. It is well known that LCs may provide huge electrically controlled refractive index changes. However, the focus tuning (which is required for optical zoom) requires the generation of spatially varying refractive index changes in the LCs, which in turn usually requires either a spatially non-uniform LC layer (e.g. a lens that is submerged in the LC cell) or a spatially varying electric field.
A simple method of obtaining a spatially varying electric field is the use of multiple (at least 3) transparent electrodes (such as Indium Tin Oxide (ITO)) distributed on the LC cell substrates. [S. T. Kowel, P. G. Kornreich, D. S. Cleverly, Adaptive liquid crystal lens, U.S. Pat. No. 4,572,616, 1986][N. A. Riza, M. C. DeJulie, Three-terminal adaptive sematic liquid-crystal lens device, Opt. Lett. 19, pp. 1013-1015, 1994] However, the fabrication of such structures requires sub-micrometer precision, their electrical driving requires rather complex electronic micro-processing and their operation is degraded by light diffraction and scattering.
Another solution that has been proposed is the combination of planar and curved electrodes, allowing the use of standard (transparent) electrodes and LC cells having two planar internal surfaces. [Liquid Crystal Lens with Spherical Electrode, B. Wang, M. Ye, M. Honma, T. Nose, S. Sato, Jpn. J. Appl. Phys. Vol. 41 (2002), pp. L1232-L1233, Part 2, No. 11A, 1 November] The non-uniform (centrally symmetric) electric field is obtained thanks to the geometrical lens-like form of the “external” curved surface which is coated by the upper electrode. In fact, the planar LC layer is sandwiched between two glass substrates. The planar ITO electrode is coated on the bottom (plane) surface of one substrate, while the second electrode is fabricated on the top of the curved zone. Such structure is difficult to fabricate and has a 0-voltage lensing property (what we call “action-at-0-voltage”), which may cause problems if an unexpected voltage failure happens.
Various geometrical solutions have been proposed to avoid the use of multiple and complex electrodes. One of them is based on the use of a two-dimensional geometrical form of electrodes. For example, a hole-patterned electrode may be used, wherein a standard cell with LC sandwiched between two substrates, the bottom of which is coated by an ITO, has a hole in the upper electrode. The application of the voltage between the upper and lower electrodes generates a centrally symmetric electric field, which reorients the LC director in a spatially non-uniform (centrally symmetric) way. The main drawback of this structure is the necessity to use very thick LC layers to be able to obtain the desired spatial profile of the electric field in the LC layer and maintain good optical quality of the lens (particularly to avoid optical aberrations).
In a completely different approach, it is also known to use the gradient of the dielectric permittivity of materials at low frequency (e.g. 1 kHz) electric field to obtain the non-uniform electric field. More specifically, an intermediate layer is inserted between two control electrodes to generate the desired gradient of the driving electric field, where this intermediate layer is made of glass and has spatially non-uniform thickness. [B. Wang, M. Ye, S. Sato, Lens of electrically controllable focal length made by a glass lens and liquid crystal layers, Applied Optics, V. 43, No. 17, pp. 3420-3425, 2004] The remaining part of the intermediate space is filled by air. The application of the low frequency electric voltage through the electrodes generates a spatially non-uniform electric field inside the LC cell, because of the non-uniformity of the dielectric permittivity of the intermediate media. The electric field in the central part of the cell will thus be different (weaker) than the electric field near the border. Unfortunately, this approach also has problems, notably the inherent 0-voltage lensing effect, the necessity of having multiple antireflection coatings to avoid high optical losses due to Fresnel reflections on multiple glass-air surfaces and the fact that the achievable contrast of the electric field is severely limited.
International Publication No. WO 2007/098602 A1 [T. Galstian, V. Presniakov, K. Asatryan, Method and apparatus for spatially modulated electric field generation and electro-optical tuning using liquid crystals, Sep. 7, 2007] discloses an improvement to the previous approach, wherein the dependence of material dielectric permittivity upon the frequency of electric field is used to obtain a LC-based tunable device. More specifically, a hidden structure, which is optically uniform but strongly non-uniform for lower frequency electric field, is inserted in the lens between the electrodes to act as an electric field modulation layer. This hidden structure fills the remaining space between the LC cell and the intermediate glass layer with a specific material having a low-frequency dielectric permittivity and a high (optical) frequency refractive index (e.g. a water-based solution, polar liquids and gels). By using in the hidden structure a combination of such a water-based solution and an intermediate material having a very low optical refractive index and low-frequency dielectric permittivity (e.g. fluorinated polymer), it is actually possible to resolve all of the prior art drawbacks described above.
A tunable optical device based on liquid crystal technology thus has many advantages over existing alternatives, including among others a planar construction. The flat transparent plates containing the liquid crystal and making up the liquid crystal layer are simple to prepare to receive the liquid crystal, as is known in the art. Since the liquid crystal responds to the electric field, and the electric field is greater when the distance between the electrodes is smaller, the flat geometry is useful in keeping the construction compact. Such flat and compact optical devices, which have no moving parts and are tunable to change optical properties, such as focus, magnification, steering angle, etc., are thus highly desirable. However, tunable optical devices based on liquid crystal technology can be expensive to manufacture. It has been discovered that the costs associated with this manufacture would decrease importantly if multiple devices could be fabricated in parallel.
One area where fabrication in parallel has been practiced to great success is in the manufacturing of semiconductor devices. Semiconductor devices are fabricated in two dimensional, planar arrays called wafers, which are only singulated in one of the final processing steps. This process is generally referred to as wafer scale processing. The singulated devices are typically connected using contact pads on the top surface to permit contact to be made from one surface of each device.
Fabrication in parallel of multiple tunable liquid crystal devices can be implemented in a similar manner as done for semiconductor devices; however, the tunable liquid crystal device has separate contacts on different levels that must be contacted (e.g. electrical connection is required between the transparent electrodes in order to power the optical properties of the liquid crystal). Several problems have been found when following a wafer scale manufacturing approach in the case of tunable liquid crystal devices. First, the contact pads may interfere with the optical device, either due to the area taken up by the contact pads or by the thickness of the contact members that can interfere with the tunable optical device being inserted into the whole lens assembly. Second, since the conductive layers are too thin to connect to from the sides of a singulated device and it is costly to bring all of the contacts to either the top or bottom surface of the device, electrical vias must be provided through process layers (e.g. glass layers) in order to reach the transparent electrode layers.
Consequently, there exists a need in the industry to provide an improved contact structure for a tunable liquid crystal optical device in order to allow for a successful parallel fabrication of multiple devices with reduced manufacturing costs.